Most aircraft gas turbine engines, as well as axial flow turbomachines used in many other applications, have within them disks or rotors which carry a multiplicity of removable blades. Such structures are used in both the compressor and the turbine parts of the engine to respectively compress and expand the working fluid. In some instances the rotating blades have shrouds at their outermost tips and are connected at these locations. More commonly, modern engines have blades that lack shrouds and are only supported at their roots in the rotor disk.
For high efficiency, it is desired to have the closest possible fit between the tips of the rotating blades and the sealing structure of the circumscribing case of the originally manufactured engine. During use, especially during the maneuvers which aircraft accomplish, there is occasionally rubbing between the blades and the circumscribing abradable sealing ring. In addition, other degradation of the blades occurs as an inevitable result of long hours of use. As a result, the clearances between the blade tips and the case are increased and it is an object of engine overhaul to restore these clearances.
To achieve good fits the rotor blades must be precisely ground to within .+-.0.025 mm so that they are all at a constant radial distance from the center line of the engine. This presents a substantial machining problem, both in original part manufacture and in overhaul.
While the tolerances sought currently are tighter than previously, there has always been a desire to have bladed rotors fit well. Primarily this has been achieved by separately machining the rotors and blades to close tolerances but this has resulted in an accumulation of tolerances greater than now is acceptable. Consequently, it is preferred to machine blades while they are a part of a disk and blade assembly. Of course, because the blades are removable they necessarily fit somewhat loosely in the rotor disk. Thus, during machining shims have been placed under the blades to thrust them radially outward to approximately the position they assume during use. Low speed workpiece rotation, common in cylindrical grinding, also tends to have the same effect, but in neither case is the seating comparable to that obtained during high speed engine rotation. Typically, horizontal rotary grinding machines have been used to grind the assemblies while they rotate at no more than a few hundred revolutions per minute.
However, the forces accompanying slow rotation or from use of shims are not powerful. It has been found that the older tip grinding processes produce a variation in length at individual blades which is too great, much more than .+-.0.025 mm on a 0.25-0.30 m diameter rotor assembly. Additionally, there is a particular rotor construction wherein the slot which receives the multiplicity of blades runs circumferentially around the rim of the disk. With this configuration, it is not practical to insert shims. Thus, during machining either the blades and their retention slot have been configured to limit radial inward movement or resilient cushions have been used to sandwich the rotor and thus capture the blades during machining. Neither of these approaches is entirely satisfactory in producing the desired accuracy.
A further problem in machining rotors involves the means for measuring the diameter of a machined rotor assembly. The diameter of the bladed rotor or fixture has traditionally been measured with the conventional gaging associated with machine shops, including in recent times the use of scanning laser micrometers. Generally, measurements are ordinarily taken when the part is not rotating although the speed of reading of various non-contact electro-optical measuring systems does permit measurements to be taken while the workpiece is rotating at a relatively low velocity of about 25 rpm. Individual blade dimensions measured under static or trivial rotational speeds will not be indicative of those in a rapidly rotating structure. There have been developed non-contact systems for measuring dimensions of high speed rotating machinery. For instance, U.S. Pat. No. 3,992,627 to Stewart discloses the use of x-rays to measure clearances in operating gas turbine engines. The technical publication co-authored by Drinkuth (one of the inventors herein) et al, "Laser Proximity Probes for the Measurement of Turbine Blade Tip Clearances" Instrumentation Society of America 20th Annual Instrumentation Symposium, May 21-23, 1974, discloses the use of a laser triangulation system to measure tip clearance in a gas turbine engine. In the Drinkuth et al system the reflected laser beam position is measured with a vidicon and cathode ray tube display. U.S. Pat. No. 4,074,104 to Fulkerson discloses a grinding method utilizing a triangulating laser beam for position measurement, where the beam is prefocused at a particular point representing the desired final dimension.
None of the foregoing measuring systems has provided anything more than an average reading of the dimensions of a rapidly rotating article. The spinning articles may have had variation in radial dimension around their circumference but the configurations of the prior systems could not output this information. Thus the dynamic readings for a bladed rotor were deficient in not revealing blade to blade variations which can cause leakage or excess wear of the circumscribing seal.
Also, if there are a number of short blades in the rotor assembly, machining to an average gage length will produce a greater than desired dimension in the longer blades. Thus there have been problems in both the actual maching procedures and the gaging procedures used in machining rotors, and the present invention is directed to their solution. The present application is related to the commonly assigned application of Miller, "Method for Cylindrical Grinding Turbine Engine Rotor Assemblies", Ser. No. 501,983, filed on even date herewith, the disclosure of which is incorporated by reference. The Miller application relates to grinding speeds and is referred to further below.